If you’ve ever observed someone’s eyes roll, or someone get shifty-eyed, you may have noticed that their eyes always move together. While people can, with great effort and a massive headache, move only one eye — there’s a reason humans move both eyes in tandem.
Each person is born with two eyes — mainly to ensure a spare, but to also ensure our brain’s ability to view things in three dimensions (3D); this skill is also known as depth perception. One eye is capable of seeing the world as a whole, but the brain co-ordinates both to achieve high-quality depth perception — which is what we know as normal vision. From an evolution perspective, this was an excellent way for hunter-gatherers to judge the relative distance between themselves and their prey.
However, to ensure that both eyes are creating that particular overlapping, singular image, they need to move together. Each eye has six muscles that must work simultaneously to move together, and the brain controls a feedback system that learns and adapts how much stimulation each muscle needs in order to move the eye around.
Related on The Swaddle:
Why We See Swirling Colors When Our Eyes Are Closed
This is a constantly adapting process because moving both eyes together isn’t an innate skill that human beings are born with. Within the first four months of life, infants learn to move their eyes together, along with learning to focus. As these eye muscles grow and develop, the brain continues to learn how to move these muscles over time, though infants master a significant amount of these movements early on. When human beings hit old age, they slowly begin to loose this ability.
Declining depth of perception due to old age, or the loss of one eye can lead to frustrating issues like being unable to distinguish road from sidewalks, and the space between staircases. It can make an individual clumsier, and unable to drive a vehicle. Fortunately, this can be corrected via bioptic lenses — regular glasses with a small telescope fitted in the center, in order to improve one’s vision.
